requestStream.Length Issue while POST

private static string PostResponse(string url, string postData, string userAgent = null)
        {
            string str = string.Empty;
            HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
            ASCIIEncoding encoding = new ASCIIEncoding();
            request.Method = "POST";
            request.ContentType = "application/x-www-form-urlencoded; charset=utf-8";
            if (userAgent != null)
            {
                request.UserAgent = userAgent;
            }
            using (Stream stream = request.GetRequestStream())
            {
                byte[] bytes = Encoding.ASCII.GetBytes(postData);
                stream.Write(bytes, 0, bytes.Length);
                stream.Close();
            }
            using (HttpWebResponse response = (HttpWebResponse)request.GetResponse())
            {
                using (Stream stream2 = response.GetResponseStream())
                {
                    using (StreamReader reader = new StreamReader(stream2))
                    {
                        str = reader.ReadToEnd();
                        reader.Close();
                    }
                    stream2.Close();
                }
                response.Close();
            }
            return str;
        }